Five participants will be initially enrolled in this unmasked pilot study. However, up to an additional three participants may be enrolled to account for participants who withdraw from the study prior to receipt of six months of study treatment. Participants will take an oral dose of 60 mg of dextromethorphan twice daily for 24 months. During each visit, participants will have their visual acuity measured and will undergo OCT testing to measure retinal thickness. Beginning at the Month 4 visit, participants will be assessed for worsening disease defined as loss of 15 ETDRS letters of vision compared to baseline or a 50% increase in total retinal thickness as measured by OCT. Additionally, beginning at the Month 6 visit, participants will be eligible for focal laser therapy unless they show marked improvement in retinal thickness and/or visual acuity or if they are not amenable to focal laser treatment. Participants who do not meet the criteria for improvement will also be eligible for anti-vascular endothelial growth factor (VEGF) treatments such as bevacizumab or ranibizumab. Participants deemed to have worsening disease will also be eligible for focal laser and/or anti-VEGF treatments. The primary outcome is the change in best-corrected visual acuity (BCVA) in the study eye at six months compared to baseline. Secondary outcomes include the change in retinal thickness as measured by OCT at 6, 12, 18 and 24 months compared to baseline, change in BCVA at 12, 18 and 24 months compared to baseline, change in mean macular sensitivity as measured by microperimetry at 6, 12, 18 and 24 months compared to baseline, as well as changes in fluid leakage in the macula as demonstrated by fluorescein angiography at 6, 12, 18 and 24 months compared to baseline. Safety outcomes include the number and severity of systemic and ocular toxicities, and adverse events.